Retroreflective sheeting in which a graphic image or other mark is built into the sheeting has found a number of important uses, particularly as distinctive labels useful to authenticate an article or document. For example, retroreflective sheetings in which legends are printed into the sheeting, as taught in U.S. Pat. Nos. 3,154,872; 3,801,183; 4,082,426; and 4,099,838, have found use as validation stickers for vehicle license plates and as security films for driver's licenses, government documents, phonograph records, tape cassettes, and the like.
Galanos, U.S. Pat. No. 4,200,875 forms directional images in "high gain retroreflective sheeting of the exposed-lens type" (col. 2, lines 3-4) by laser irradiation of the sheeting through a mask or pattern. The noted "high gain retroreflective sheeting" comprises "a plurality of transparent glass microspheres . . . that are partially embedded in a binder layer . . . and that are partially exposed above the binder layer, . . . with a reflective layer . . . adjacent to and disposed behind the embedded surface of each of the plurality of transparent glass microspheres" (col. 1, lines 25-32). The "binder layer, such as 12, FIG. 1, is pigmented in carbon black" (col. 1, line 55). When such "high gain retroreflective sheeting" is irradiated while positioned at a specific angle to a laser light beam, an image is formed which "can be seen thereafter with the naked eye if, and only if, the target (sheeting) is again positioned at that same angle" (col. 2, lines 9-11). When "a plurality of different preselected patterns are individually and separately recorded, at different angles, . . . then each of the recorded patterns is visible only when viewed at the angle at which it was recorded by the laser light beam radiation on the target" (col 2, lines 12-18).
Although the Galanos patent states that the sheeting he used is illustrated in FIG. 1, the pigmented under layer 12 would have prevented one from seeing any images except at approximately 90.degree. to the face of the sheeting. However, the "Scotchlite" brand reflective sheetings "High Gain" No. 7610 and No. 7611, which apparently were used, were of substantially different construction. That is, the back hemisphere of each glass microsphere had a specularly reflective layer and its front hemisphere was open, thus permitting directional images to be formed over a wide range of angles.
The Galanos patent does not explain how a directional image is formed in the described retroreflective sheeting target other than to state that the laser preferably is "a high energy one of the Q-switched type having a power (energy?) of one Joule" and that its beam preferably "has a diameter of 5/8 inch and a pulse width of 90 nanoseconds" (col. 3, lines 34-38). The patent points out that the pattern of the image must be smaller than the target, but "that the diameter of the beam 22 as compared to the size of the pattern is immaterial, because even if the laser light beam (radiation) 22 is smaller than the pattern, the laser 21 can be moved in a scanning action" (sentence bridging col. 3 & 4). Galanos says that the directional image is "recorded on the target by structural alteration, i.e., modification of the target material" (col. 4, lines 32 and 51). From a bare reading of the Galanos patent, one skilled in the art would not know what constituent of the retroreflective sheeting material had been structurally altered. Based on our work in making the present invention, a likely explanation for the image formation observed by Galanos is a modification of the glass microspheres, e.g., by a localized devitrification, melting, or erosion of the glass at the back edges of the microspheres where the laser beam is focused.
Some advantages of the imaged sheeting taught in the Galanos patent are that the images are seen within sharply defined angular ranges, which could be used as an identifying characteristic to help assure that the sheeting is genuine; and the images can be formed in an already manufactured retroreflective sheeting. As compared to procedures of the four patents cited in the first paragraph of this "Background Art" section, the Galanos directional images can be formed in an already manufactured retroreflective sheeting, thereby avoiding the need for inventories of specially printed retroreflective sheeting and the need for long process runs to distribute the costs in setting up a printing operation.
However, the directionally imaged sheeting taught in the Galanos patent also has important disadvantages which limit its utility. For example, it is not useful to provide retroreflective images in outdoor environments where it may receive precipitation, because such precipitation can alter the optical relationships in the sheeting and obliterate or greatly reduce a retroreflective image.